1. Field of the Invention
The present invention relates to a magnetic head with a precisely defined zero throat height (ZTH) and more particularly to a magnetic head that employs a thin strip of baked photoresist for defining the ZTH.
2. Description of the Related Art
A merged magnetic head includes a write head portion and a read head portion. The write head portion includes a coil layer embedded in first, second and third insulation layers (called "the insulation stack"), the insulation stack being located between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted through the coil layer produces a magnetic field in the pole pieces. The magnetic field fringes across the gap at the ABS for the purpose of writing information in the form of magnetic impressions in tracks on moving magnetic media, such as in circular tracks on a rotating magnetic disk or in longitudinal tracks on a moving magnetic tape.
The read head portion of the merged head includes a read sensor that is sandwiched between first and second gap layers. The first and second gap layers are sandwiched between first and second shield layers. The first and second gap layers magnetically insulate the read sensor from the shield layers and the shield layers protect the read sensor from stray magnetic fields. The read sensor may be an anisotropic magnetoresistive (AMR) sensor or a spin valve sensor. In either instance a recessed edge of the sensor is referred to in the art as the "stripe height" of the read head. This height is important because it establishes the magnetics of the read head. Flux signals traversing the sensor from a rotating magnetic disk causes a change in resistance in the sensor that is detected by processing circuitry when a sense current is conducted through the sensor.
An important parameter in the design of the write head is the location of the zero throat height (ZTH). The zero throat height is the location where the first and second pole piece layers first commence to separate from one another after the ABS. Flux leakage between the first and second pole piece layers is minimized by locating the zero throat height as close as possible to the ABS. Short zero throat heights have been limited by prior art methods of construction.
In the prior art, the forward sloping edge of one of the first, second or third insulation layers of the insulation stack is employed for defining the zero throat height. It is important that the zero throat height be accurately located relative to the height of the stripe of the read head. When a partially completed merged head is lapped to the specified air bearing surface the write head should have the desired zero throat height and the stripe should have the desired stripe height in order to satisfy the designed magnetics of the head. It is also important that the zero throat defining insulation layer have a forward edge at the zero throat height that slopes at an appropriate angle, which is referred to in the art as the apex angle. The first insulation layer of the insulation stack can be relatively thin which results in a low apex angle. In contrast the second insulation layer of the insulation stack is a relatively thick layer which results in a higher apex angle. A higher apex angle, such as 35 degrees, is desirable for several reasons: (1) a lower apex angle results in more flux leakage between the first and second pole piece layers and (2) a lower apex angle results in more variability (windage) in the location of the forward edge of the zero throat defining insulation layer due to subsequent processing steps.
Each insulation layer of the insulation stack is constructed of photoresist. Photoresist is spun on and planarized across a wafer where multiple magnetic heads are to be constructed. For each head, the photoresist is photopatterned by light imaging so as to prepare portions of the photoresist for removal by developing. The photoresist is then developed leaving a photoresist layer with desired openings. The photoresist layer is then baked at a high temperature which causes it to shrink and solidify. Each of the insulation layers of the insulation stack is constructed one on top of the other commencing with the first insulation layer.
The longer the insulation layer the more the forward edge of the insulation will recess into the head due to shrinkage of the layer. When the zero throat defining insulation layer is the first insulation layer of the insulation stack it is subjected to process variations during subsequent construction of the coil layer. After the coil layer is frame plated a seedlayer is removed by sputter etching, ion milling or the like which also etches or ion mills the forward edge of the first insulation layer. This causes the forward edge of the first insulation layer to be relocated further into the head. A thin first insulation layer causes the forward edge of the layer to have a low apex angle. Unfortunately, etching or ion milling removes more of the forward edge of the layer when the apex angle is small because the full height of the layer is further back in the head. As stated hereinabove a low aspect angle also causes more flux to leak between the first and second pole piece layers.
When the second or third insulation layer of the insulation stack is selected for the zero throat defining insulation layer the forward edge has a higher aspect angle which is more favorable for reducing flux leakage between the first and second pole tip layers. Further the second and third insulation layers are not subject to etching or ion milling since they are constructed after construction of the coil layer. Unfortunately, however, the second or third insulation layers can be relatively thick and shrinkage during baking causes the forward edge of the insulation layer to be further recessed in the head than when the first insulation layer is used as the zero throat defining layer.
Accordingly, defining the zero throat height with any of the insulation layers of the insulation stack has not been satisfactory. The windage or relocation of the forward edge of the selected insulation layer during construction has made the exact location of the zero throat height unpredictable. The relative location of the zero throat height to the stripe height after lapping is then uncertain. There is a strong felt need for construction of the zero throat height insulation layer that results in a more predictable location and aspect angle of the forward edge of the layer.